This invention relates to stay rings (also called speed rings) for the spiral casings of rotary hydraulic machines, such as hydraulic turbines, pump turbines and pumps.
A stay ring comprises generally two ring members disposed coaxially about the machine axis and spaced from each other axially of the machine, and axially extending stay vanes connected between said ring members. The ends of the metal plates of the spiral casing are joined to said ring members. Such a stay ring has as its objects to form a flow path for the fluid flow between the interior of the spiral casing and the guide vane ring, which is located radially inwardly of the stay ring, to secure the inner edges of the spiral casing and to absorb axial load forces from the guide vane ring and the turbine cover and sometimes also concrete weights.
Such stay rings are subjected to considerable forces originating, primarily, from the internal hydrostatic pressure acting on the walls of the spiral casing that is joined to the stay ring and, to a lesser extent, from the pressure within the stay ring itself and from the guide vane ring and/or the turbine cover which are connected to the stay ring. The forces due to loads imposed on the stay ring can be resolved, for purposes of analysis, into radial components perpendicular to the axes of the machine and axial components. The radial components produce internal circumferential tensile stresses in each of the two ring members of the stay ring, whereas the axial components produce internal axial tensile stresses in the stay vanes. It is important that the internal stresses in the various parts of the stay ring be distributed as uniformly as possible over the cross sections of these parts, in order to achieve an optimal utilization of the material and thereby reduce the dimensions of the parts and obtain substantial economical advantages as to the costs for the material and also the transport of the stay ring from the factory to the site of installation. In this connection, it should be noted that a reduction of the radial width of the stay ring results in a corresponding reduction of the total dimensions of the spiral casing located radially outside of the stay ring, which is also advantageous with respect to cost, weight, and transport.
Inasmuch as all forces acting upon the stay ring are initially imposed upon the two ring members of the stay ring, it is essential that the ring members and the connections between the walls of the spiral casing and the ring members and also between the ring members and the stay vanes be of such a design that the ring members can absorb such forces without any substantial bending, twisting or other distortion of the ring members and also transfer the axial components of such forces to the stay vanes as uniformly distributed as possible over the cross sections of the stay vanes.
With the foregoing objectives, stay rings of a welded construction have been suggested in the prior art (for instance in German Pat. Nos. 1 066 149 and 1 258 360 and Swedish Pat. No. 221 458). In the designs of these prior art welded stay rings, one objective has been to direct the forces from the spiral casing through the center of gravity of the cross section of the ring members and, in turn, to make the axial components of all external forces acting upon the ring members coincide as much as possible with the axes of the stay vanes. However, these known, so-called "balanced" stay rings of welded construction have several serious disadvantages. For example, in many of these prior art stay rings the ring members include metal plates which are subjected to very large tensile stresses in a direction perpendicular to the plane of the plates. It is well known that rolled metal plate inherently has a somewhat laminated structure parallel to its plane, wherefore there is a considerable risk of delamination of a metal plate when it is subjected to tensile stresses perpendicular to its plane. Moreover, there are often considerable stress concentrations at certain locations in many known stay rings. Also, in many of the prior art structures the ring members have an unsatisfactory rigidity or stiffness relative to bending and twisting or are of a comparatively complicated design, thus necessitating large amounts of welding, which is time consuming and expensive. In many cases, large parts of the welds are located in places that are not readily accessible, wherefore it is difficult to make the welds and check their quality in a satisfactory way.